The present invention generally relates to hardcopy apparatus, such as copiers, printers, scanners, facsimiles, and more particularly to improved media holddown devices for such apparatus.
To reduce the effects of paper curl and cockle on dot placement during printing, conventional practice is to employ sheet holddown devices such as electrostatic or suction devices. Cockle effect is the reluctance of the paper to bend smoothly. Instead it bends locally in a sharp fashion, creating permanent wrinkles.
In an electrostatic holddown device, for example, paper flatness is maintained by establishing electrostatic attraction between a flat support plate on the printer and the back surface of a sheet to be printed. Likewise, in vacuum holddown devices, sheet flatness is maintained by providing suction between a support plate and the back surface of a sheet to be printed. It should be noted that, in either type of holddown device, direct contact of the holddown device with the printed surface is avoided to minimise ink smearing and other adverse affects on print appearance.
Although conventional vacuum holddown devices are fairly effective in maintaining sheet flatness during printing, they have drawbacks. One drawback is the complexity of maintaining the same holddown force along the entire width of the medium while printing, i.e. in the direction of the printheads motion. This is due to the losses of air that the conventional devices allow, causing the medium to be subject to different forces, i.e. forcing the medium to rotate while it is advanced in the direction of the media motion.
Another drawback is that on one hand the maximum holddown force on a sheet is limited because of the necessity to maintain low frictional loads on transport devices which index the sheets. In conventional inkjet printers, such limitations can cause pen-to-sheet spacing distances to vary from swath to swath. Consequently, the holddown pressure at a localised area being printed may be insufficient to flatten cockles and other paper irregularities. On the other hand the vacuum required to eliminate cockle wrinkles in a printout would be so high that is normally unfeasible; in fact, high vacuum may suck the ink right through the paper and at the same time generate a lot of noise.
The present invention seeks to provide an improved hardcopy apparatus and method of holding down a medium in the hardcopy apparatus.
According to an aspect of the present invention there is provided a hardcopy apparatus comprising a media holddown unit and a independent media drive roller, such holddown unit comprising a platen, on which a print zone is defined, and a vacuum source for generating a negative pressure for holding at least a portion of a medium substantially flat on said print zone, said holddown unit having said platen extending towards, and partially overlapping, said drive roller.
This means that since the drive roller is not comprised in the holddown unit, particularly it has not been included into the vacuum channel of the holddown unit, the amount of air losses is considerably reduced. Whilst having the platen extending towards the drive roller allows to define the print zone closer to the drive roller itself with all the consequent benefit in terms of increased accurateness during the indexing of media and improved capability of printing closer to the edges of a cut sheet.
In addiction, the present invention can be particularly suitable to inkjet printers which preferably require a media to be periodically accurately indexed across a print zone defined in the printer for receiving the ink.
Preferably, the vacuum source is connected to the atmosphere through a vacuum channel and a plurality of first apertures, said holddown unit further comprises means for extending the negative pressure, generated at the plurality of first apertures, to a position closer to said drive roller. In a preferred embodiment, said means for extending the negative pressure comprise a plurality of grooves extending towards the drive roller and a number of said plurality of first apertures are located within said grooves. Moreover, each aperture of said plurality of first apertures is located within a groove.
Thus, the medium can be maintained substantially flat in a position closer to the drive roller. Since it is important to maintain the medium flat particularly in the print zone, the same can be now defined closer to the drive roller.
Typically, a portion of said print zone is overlapping at least part of said grooves. Accordingly, the medium is kept more adherent to the platen, and so flat, particularly in the print zone.
However, high vacuum may crease the paper especially if the grooves of the slot are wide and run parallel to the paper advance direction. In addition if the groove is parallel to the advance direction, it may make the ink to migrate and create localised dark areas.
More preferably, said plurality, of grooves are oriented at substantially 45xc2x0 respect a direction of motion of media through the hardcopy apparatus, and said plurality of grooves alternates grooves located alternately substantially at 45xc2x0 either side of the media direction. Accordingly, running the grooves at about 45xc2x0 helps in reducing the above stated drawbacks and furthermore evenly distribute the vacuum along the print zone.
However, an interrupted sequence of grooves may create areas, having a reduced vacuum, which cross the complete print zone, in the media axis direction. This may force the ink applied in those areas to migrate and create localised dark or clear portions in the printout.
In a further preferred embodiment, said plurality of grooves are linked together to generate one or more substantially continuous slots.
Accordingly, the continuous shape of the waved slot help the system to evenly distribute the vacuum along the print zone and to reduce the occurrence of undesired migration of the ink over the medium.
Preferably, at least one of said plurality of first apertures is located substantially at the end of a groove further from the drive roller.
It has been noted how the paper works in compression, some very thin papers may even buckle and create loops between the drive roller and the print zone.
Accordingly, in an enhanced embodiment, said holddown unit further comprises first vacuum-controlled means for tensioning the medium. Furthermore, overdrive forces, i.e. tensioning the paper in the feeding direction, can reduce the height reached by the cockle wrinkles by as much as a half.
In any hardcopy apparatus and particularly in apparatus employing sheet of big size, such as large format printers, it appears to be cumbersome the manual loading of a cut sheet of media. In fact the manual loading conventionally implies the use of some mechanism to hold the medium to be manually actuated once it is properly positioned, or which may be automatically actuated even if the sheet has not been accurately positioned yet.
In a preferred arrangement said holddown unit further comprises second vacuum-controlled means for feeding the hardcopy apparatus with a cut sheet of media. This provides the same holddown unit with further capabilities, so avoiding the need of employing further holddown systems. Particularly allows an easier way of manually loading a cut sheet of media. Furthermore it allows to stably hold the medium without the need of any element to be placed in contact with the side of the sheet to be printed on.
Advantageously, said holddown unit further comprises third vacuum-controlled means for outputting printed media from the print area, and holding means for holding printed media for a predetermined dry time.
This provides the same holddown unit with further capabilities, so avoiding the need of employing further holddown systems. Particularly it is now possible to avoid the use of starwheels for outputting the printed medium, which may damage the quality of the printout.
Viewing another aspect of the present invention, there is also provided a method for holding at least a portion of a medium to be printed on substantially flat over a print zone of a hardcopy apparatus comprising the following steps:
indexing the medium (over said print zone;
generating a negative pressure capable of holding a portion of the medium substantially flat on the print zone;
extending the negative pressure generated to the print zone, in order to achieve a substantially uniform holding force over the print zone.
Preferably, the method further comprises the step of tensioning the medium.